Biohydrogen production from lactic acid: Use of food waste as substrate and evaluation of pretreated sludge and native microbial community as inoculum

Abstract

Nowadays, lactic acid (HLa) has caught the eye as a precursor of H2 production from substrates that harbor a significant amount of lactic acid bacteria (LAB), such as Lactobacillus and Lactococcus. Since the native microbial community of food waste (FW) is dominated by LAB, the production of H2 during dark fermentation might be inhibited by the displacement of H2-producing bacteria by LAB. Therefore, lactate-driven dark fermentation in two steps is an alternative to overcome the overproliferation of LAB. In this process, FW is converted into HLa, which can subsequently be transformed into H2 by HLa-consuming bacteria during a cross-feeding mechanism with H2-producing bacteria. In the present study, FW was evaluated for HLa production at different organic loading rates (OLR) (12.5, 25.0, 37.5, and 50.0 gVS/L/d) in a semi-continuous reactor, and the HLa obtained was used for the H2 production. Results showed that OLR significantly influences HLa production during the self-fermentation of FW (p < 0.05). The highest ORL of 50.0 gVS/L/d and an HRT of 1.25 days generated the net maximum HLa production of 10.8 g/L. The HLa production was companied by the presence of acetate, propionate, and succinate, which accounted for only 20 % of the total organic acids produced. Then, effluent with HLa concentrations of 5.0, 10.0, 15.0, 20.0, and 25.0 g/L was evaluated for H2 production in batch tests using thermally pretreated sludge and the native microbial community of the effluent as inoculum. Even though different metabolic pathways for H2 production can be noted during the fermentation, which differs from one inoculum to another, the highest value of H2 production of 596.3 mL/Lreactor was obtained with the native microbial community at the highest concentration of HLa (25.0 g/L). During the fermentation process for H2 production, HLa and propionate were mainly transformed into butyrate and acetate. The present study demonstrated the feasibility of obtaining H2 from a complex substrate, such as effluent from a previous fermentation, using different inocula.